1. Field of the Invention
The invention relates to cordierite ceramic bodies for use as catalyst carriers, particularly to cordierite bodies, having an ultra low thermal expansion, for use as catalyst carriers for purifying automobile exhaust gas, and a method for producing the cordierite structures.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970's.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to cordierite ceramics' high thermal shock resistance. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have a good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application. It is generally known that the coefficient of thermal expansion of cordierite bodies is about 18.times.10.sup.-7 /.degree. C. in the range of 25.degree. C.-800.degree. C. for those polycrystalline cordierite bodies in which the cordierite crystals are randomly oriented.
The production of cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) ceramics from mineral batches containing sources of magnesium, aluminum and silicon such as clay and talc is well known. Such a process is described in U.S. Pat. No. 2,684,919. U.S. Pat. No. 3,885,977 discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along at least one direction. Furthermore, this reference describes the principle of orienting the cordierite crystals with their crystallographic c-axis in the plane of the honeycomb webs, resulting in thermal expansion values as low as 5.5.times.10.sup.-7 /.degree. C.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to optimize the thermal shock resistance and strength of the cordierite substrates. The following patents each relate to the manufacture of ceramic honeycombs exhibiting improved thermal shock resistance or coefficient of thermal expansion (CTE).
U.S. Pat. No. 4,434,117 (Inoguchi et al.) discloses the use of a raw material mixture comprising plate-shaped talc particles and non-plate shaped particles of other ceramic materials and thereafter anisostatically forming the mixed batch so as to impart a planar orientation to the plate-shaped talc particles and then drying and firing to obtain a formed ceramic body. The ceramic bodies formed in the Inoguchi reference exhibited thermal expansion coefficients as low as 7.0.times.10.sup.-7 /.degree. C.
U.S. Pat. Nos. 5,144,643 (Beall et al.) and 5,144, 643 (Beall et al.) disclose a method of fabricating a cordierite body involving selecting specific raw materials that will form the desired cordierite body. Specifically, these raw material selections should not include any clay or talc, should include a MgO-yielding component and an Al.sub.2 O.sub.3 -yielding component having a particle size of no greater than 15 and 8 .mu.m, respectively. The raw materials are mixed together, subsequently dried and fired for a time and a temperature sufficient to form the aforementioned cordierite body. The ceramic bodies formed by these Beall references exhibited thermal expansion coefficients of less than about 9.times.10.sup.-7 /.degree. C. from about 25 to about 1000.degree. C.
Lastly, U.S. Pat. No. 5,258,150 (Merkel et al.) discloses a method of forming a cordierite body involving a raw material batch mixture comprising certain selected raw materials including platy talc, 0 to 48% of a platelet-type or delaminated clay and an aluminum oxide-yielding component having an average particle size of between 3 to 8 .mu.m, or less than 3 .mu.m. The method involves mixing the raw materials with a binder system and extruding the mixture to form a green body and thereafter firing the green body at a temperature of at least 1390.degree. C. to result in a sintered cordierite body. The ceramic bodies formed by this Merkel reference exhibited thermal expansion coefficients of less than about 4.times.10.sup.-7 /.degree. C. from about 25 to about 1000.degree. C., a porosity greater than about 42% and a median pore diameter of between about 5 to 40 .mu.m; however, these bodies are disclosed therein as exhibiting I-ratios of no greater than about 0.91.
While such ceramics represent an improvement in the thermal expansion coefficient properties over extruded cordierite ceramics produced using pre-existing processes, still further improvements in this thermal expansion characteristic, particularly without a measurable reduction in the ceramics' strength would be desirable. Strength has become an increasingly important consideration in the production of cordierite honeycomb substrates as a result of the move to producing thinner-walled, higher cell density, increased catalytic conversion efficiency and lower back pressure cordierite honeycomb catalyst carriers.
It is therefore a principal object of the present invention to provide improved cordierite ceramics, and method for making them, that exhibit adequate strength in combination with an ultra-low thermal expansion.